1. Field of the Invention
The present invention relates to a compression volume control apparatus for a refrigeration cycle particularly for use in an air-conditioning system of a vehicle, including a variable displacement refrigerant compressor having a suction chamber connected to a low-pressure refrigerant pipe and a refrigerant discharge chamber connected to a high-pressure refrigerant pipe (2), a refrigerant discharge volume being variable by varying the pressure in a pressure adjusting chamber of said compressor.
As the compressor used in a refrigeration cycle of a vehicular air-conditioning system directly is coupled to the engine by a belt, the speed of the compressor cannot be controlled independently. In order to achieve an adequate cooling performance without a restriction by the momentary engine speed, it is conventional to use a variable displacement compressor, the compression volume or discharge volume of which can be altered.
Different types of variable displacement compressors can be used like the so-called swash plate type, the rotary type and the scroll type. By way of an example the swash plate type compressor will be explained here. It operates with reciprocating pistons by rotating a driving oscillating plate provided in the crank chamber. The stroke of the pistons is varied by varying the inclination angle of said plate with respect to a driving shaft.
In a swash plate compressor for variable displacement the crank chamber defines a pressure adjusting chamber to vary the displacement of the compressor for compression volume control. The crank chamber pressure conventionally is controlled in association with a change of a suction pressure in order to vary the volume.
When controlling the volume based on the suction pressure, however, a flexible film member like a diaphragm or bellows responding to pressure variations is used which is placed in a moveable manner in the compression volume control apparatus. For that reason the apparatus has to be designed large and the costs for the apparatus are high.
2. Discussion of the Related Art
Another volume control apparatus as known from Japanese Laid-Open patent publication No. Hei 5-87047 is provided with an electromagnetic control valve for interconnecting or separating the crank chamber defining the pressure adjusting chamber and the suction chamber to maintain a differential pressure between the crank chamber pressure and the suction pressure at a predetermined value, e.g. as selected by the adjusted electromagnetic force and by spring forces. The electromagnetic force of the electromagnetic control valve is changed to change the value of said differential pressure as well. The structure of said control valve is simple and compact. The apparatus costs are fair.
FIG. 6 is a line chart showing the xe2x80x9cEnthalpy versus Refrigerant Pressurexe2x80x9d characteristics of a refrigeration cycle. The displacement of the compressor is controlled on the basis of a differential pressure Pcxe2x88x92Ps between the crank chamber pressure Pc and the suction pressure Ps. The discharge pressure Pd is then changed accordingly which automatically leads to further change of the differential pressure Pcxe2x88x92Ps. Said control routine is repeated under feedback control via the entire refrigeration cycle as a system. Said control routine has a shortcoming because a time delay occurs for the discharge volume to reach a predetermined value when the electromagnetic force of the electromagnetic control valve is changed. The result is that the compression volume control cannot be carried out promptly enough.
It is an object of the present invention to provide a fast responding compression volume control apparatus for a refrigerating cycle which allows to achieve a predetermined compression volume promptly and without a time delay as soon as the electromagnetic force of the electromagnetic control valve is changed.
Said electromagnetic control valve connects or separates said pressure adjusting chamber from said discharge chamber or the suction chamber in order to maintain the differential pressure between at least one of the pressure in the pressure adjusting chamber and the pressure in the suction chamber at one side and the pressure in the discharge chamber at the other side at a predetermined differential pressure value. Said differential pressure value is changed by changing the electromagnetic force of the electromagnetic control valve in order to control the discharge volume of the refrigerant. The control routine is executed on the basis of the level of the discharge pressure Pd itself which in turn is changed by volume control and feedback control only carried out by the compressor portion. As soon as the electromagnetic force of the electromagnetic control valve is changed the compression volume promptly reaches a predetermined value without a time delay. This ensures a fast response compression volume control.
In a first preferred embodiment said electromagnetic control valve exclusively is establishing a connection or separation between the discharge chamber and the pressure adjusting chamber, the pressures in said discharge chamber and said pressure adjusting chambers both are acting counter to said electromagnetic force loading said valve body in closing direction. For decreasing said pressure in said pressure adjusting chamber a leakage passage is provided between said pressure adjusting chamber and said low-pressure suction pipe.
In another preferred embodiment the pressure in said discharge chamber is loading said valve body in opening direction and counter to the electromagnetic force, while said suction chamber pressure is loading said valve body in closing direction. Said pressure in said pressure adjusting chamber has no influence on the loading of the valve body in either direction. The necessary leakage path, e.g. between said suction chamber and said pressure adjusting chamber, can be provided inside said electromagnetic control valve.
In another preferred embodiment the pressure in said discharge chamber is loading said valve body in the same direction as said electromagnetic force and counter to the pressure in said pressure adjusting chamber, while the pressure in said suction chamber does not have any influence on the motion of said valve body.
In another preferred embodiment the pressure in said discharge chamber is loading said valve body in closing direction and in parallel with said electromagnetic force, while said pressure in said pressure suction chamber is loading said valve body in opening direction and counter to said electromagnetic force. The pressure in said pressure adjusting chamber has no influence on the motions of said valve body.
In a further preferred embodiment a valve moveable between an open and closed position is provided in the low-pressure suction line upstream of said suction chamber. Said valve is pilot operated by an auxiliary valve situated within said electromagnetic control valve. Said pilot valve is actuated by said electromagnetic control valve in order to open and close said valve in said low-pressure pipe.